The present invention relates to an apparatus and method for measuring a process variable. More particularly, the present invention relates to an improved method for providing an accurate indication of the location of an interface between a first medium and a second medium in a vessel using time-of-flight of signal reflections, and methods for detecting and correcting or reporting potential conditions effecting process variable measurement.
The process and storage industries have long used various types of equipment to measure process parameters such as level, flow, temperature, etc. A number of different techniques (such as mechanical, capacitance, ultrasonic, hydrostatic, etc.) provide measurement solutions for many applications. However, many other applications remain for which no available technology can provide a solution, or which cannot provide such a solution at a reasonable cost. For many applications that could benefit from a level measurement system, currently available level measurement systems are too expensive.
In certain applications, such as high volume petroleum storage, the value of the measured materials is high enough to justify high cost level measurement systems which are required for the extreme accuracy needed. Such expensive measurement systems can include a servo tank gauging system or a frequency modulated continuous wave radar system.
Further, there are many applications that exist where the need to measure level of the product is high in order to maintain product quality, conserve resources, improve safety, etc. However, lower cost measurement systems are needed in order to allow a plant to instrument its measurements fully.
There are certain process measurement applications that demand other than conventional measurement approaches. For example, applications demanding high temperature and high pressure capabilities during level measurements must typically rely on capacitance measurement. However, conventional capacitance measurement systems are vulnerable to errors induced by changing material characteristics. Further, the inherent nature of capacitance measurement techniques prevents the use of such capacitance level measurement techniques in vessels containing more than one fluid layer.
Ultrasonic time-of-flight technology has reduced concerns regarding level indications changing as material characteristics change. However, ultrasonic level measurement sensors cannot work under high temperatures, high pressures, or in vacuums. In addition, such ultrasonic sensors have a low tolerance for acoustic noise.
One technological approach to solving these problems is the use of guided wave pulses. These pulses are transmitted down a dual probe transmission line into the stored material, and are reflected from probe impedance changes which correlate with the fluid level. Process electronics then convert the time-of-flight signals into a meaningful fluid level reading. Conventional guided wave pulse techniques are very expensive due to the nature of equipment needed to produce high-quality, short pulses and to measure the time-of-flight for such short time events. Further, such probes are not a simple construction and are expensive to produce compared to simple capacitance level probes.
Recent developments by the National Laboratory System now make it possible to generate fast, low power pulses, and to time their return with very inexpensive circuits. See, for example, U.S. Pat. Nos. 5,345,471 and 5,361,070. However, this new technology alone will not permit proliferation of level measurement technology into process and storage measurement applications. The pulses generated by this new technology are broadband, and also are not square wave pulses. In addition, the generated pulses have a very low power level. Such pulses are at a frequency of 100 MHz or higher, and have an average power level of about 1 nW or lower. These factors present new problems that must be overcome to transmit the pulses down a probe and back and to process and interpret the returned pulses.
The reflected pulses can include reflections that interfere with the determination of the fiducial used in alignment of the reflected pulse for measurement of the process variable. If the wrong point is selected as the fiducial or if the fiducial varies from measurement to measurement, the system will produce erroneous results for the measurement of the process variable.
The process variable to be measured may be undetected for various reasons, including a broken probe, low amplitude reflections from the material level, loss of high frequency connection and an empty vessel. The first three of these conditions result in erroneous level measurements which must be corrected whereas the latter is a valid level measurement condition. It is important to be able to detect these conditions and differentiate between them so as to avoid erroneous results.
The process variable to be measured may produce a reflection pulse which is similar in amplitude to other pulses of the reflection signal that are unrelated to the process variable to be measured. The system must be able to determine which reflection pulse is due to the process variable to be measured in order to avoid erroneous results.
Accordingly, a need exists for a method of automatically updating the reference signal on a periodic basis to track the reflections due to factors which are unrelated to the level of material in the vessel. Thereby allowing the detection of the reflection due to the material level and the accurate reporting of the appropriate process variable.
First, a sensor apparatus must be provided for transmitting these low power, high frequency pulses down a probe and effecting their return. Such appropriate sensor apparatus is described in U.S. Pat. No. 5,661,251 entitled SENSOR APPARATUS FOR PROCESS MEASUREMENT and U.S. Pat. No. 5,827,985 entitled SENSOR APPARATUS FOR PROCESS MEASUREMENT, the disclosures of which are hereby expressly incorporated by reference into the present application.
The sensor apparatus is particularly adapted for the measurement of material levels in process vessels and storage vessels, but is not limited thereto. It is understood that the sensor apparatus may be used for measurement of other process variables such as flow, composition, dielectric constant, moisture content, etc. In the specification and claims, the term xe2x80x9cvesselxe2x80x9d refers to pipes, chutes, bins, tanks, reservoirs or any other storage vessels. Such storage vessels may also include fuel tanks, and a host of automotive or vehicular fluid storage systems or reservoirs for engine oil, hydraulic fluids, brake fluids, wiper fluids, coolant, power steering fluid, transmission fluid, and fuel.
The present invention propagates electromagnetic energy down an inexpensive, signal conductor transmission line as an alternative to conventional coax cable or dual transmission lines. The Goubau line lends itself to applications for a level measurement sensor where an economical rod or cable probe (i.e., a one conductor instead of a twin or dual conductor approach) is desired. The single conductor approach enables not only taking advantage of new pulse generation and detection technologies, but also constructing probes in a manner similar to economical capacitance level probes.
The present invention specifically relates to a signal processor apparatus for processing and interpreting the returned pulses from the conductor. Due to the low power, broadband pulses used in accordance with the present invention, such signal processing to provide a meaningful indication of the process variable is difficult. Conventional signal processing techniques use only simple peak detection to monitor reflections of the pulses.
The present invention provides signal processing circuitry configured for measurement of the time-of-flight of very fast, guided wave pulses. Techniques used in similar processes, such as ultrasonic level measurement are vastly different from and are insufficient for detection of guided electromagnetic wave pulses due to the differences in signal characteristics. For example, ultrasonic signals are much noisier and have large dynamic ranges of about 120 dB and higher. Guided electromagnetic waves in this context are low in noise and have low dynamic ranges (less than 10:1) compared to the ultrasonic signals, and are modified by environmental impedances. The signal processor of the present invention is configured to determine an appropriate reflection pulse of these low power signals from surrounding environmental influences.
Standard electromagnetic reflection measurements are known as time domain reflectometry (TDR). TDR devices for level measurement require the measuring of the time of flight of a transit pulse and a subsequently produced reflective pulse received at the launching site of the transit pulse. This measurement is typically accomplished by determining the time interval between the maximum amplitude of the received pulse. The determination of this time interval is done by counting the interval between the transmitted pulse and the received pulse.
The present invention provides an improved signal processor for determining a valid reflective pulse signal caused by an interface of material in contact with a probe element of a sensor apparatus. The processor apparatus of the present invention is particularly useful for processing high speed, low power pulses as discussed above. In the preferred embodiment of the signal processor apparatus, processing is performed based on a digital sampling of an analog output of the reflective pulses. It is understood, however, that similar signal processing techniques can be used on the analog signal in real time.
The present invention provides a method for processing a time domain reflectometry (TDR) signal having a plurality of reflection pulses to generate a valid output result corresponding to a process variable for a material in a vessel. The method includes the steps of determining a reference signal along a probe in the vessel and establishing a reference end of probe location using the reference signal. The method also includes the steps of periodically detecting a TDR signal along the probe, determining a detected end of probe location on said TDR signal, determining a system status based upon the difference between the reference end of probe location and the detected end of probe location, and computing the output result when the system status is functional.
The present invention provides a method for aligning the reference signal and the time domain reflectometry (TDR) signal for the computation and comparison of distances and locations. The method includes the steps of establishing a first fiducial reference point on the reference signal and establishing a second fiducial reference point on the TDR signal. The distances and locations on the reference signal are computed relative to the first fiducial reference point and the distances and locations on the TDR signal are computed relative to the second fiducial reference point. One method of establishing a fiducial reference point includes the steps of detecting the reflection in the signal having the greatest number of consecutive zero values; and establishing the fiducial reference point as the point where the reflection first crosses a fiducial threshold. An alternative method of establishing a fiducial reference point includes the steps of detecting the reflection in the signal representing the greatest uninterrupted distance of zero values; and establishing the fiducial reference point as the point where the reflection first crosses a fiducial threshold. The preferred method of establishing a fiducial reference point includes the steps of detecting the rightmost reflection in the signal having a greater width of zero values than a fiducial width threshold; and establishing the fiducial reference point as the point where the reflection first crosses a fiducial threshold.
One aspect of the present invention is the capability of detecting a broken cable condition. The method includes the steps of establishing a measuring length which is less than the reference end of probe location. A broken cable condition is detected when the detected end of probe location is less than the measuring length.
Another aspect of the present invention is the capability of detecting a loss of high frequency connector condition. The method includes the steps of establishing an end of probe peak to peak threshold, detecting an end of probe negative peak and an end of probe positive peak on the TDR signal, and computing an end of probe peak to peak amplitude as the difference between the end of probe negative peak and the end of probe positive peak. A loss of high frequency connection condition is detected when the end of probe peak to peak amplitude is less than the end of probe peak to peak threshold.
Yet another aspect of the present invention is the capability of detecting a low amplitude level reflection condition. The method includes the steps of establishing a maximum probe length which is greater than the reference end of probe location. A low amplitude level reflection condition is detected when the detected end of probe location is greater than or equal to the maximum probe length and no level reflection was detected.
A further aspect of the invention is to only indicate the low amplitude level reflection condition if it occurs over an extended period of time without an intervening level reflection being detected or an empty vessel condition being detected.
Yet a further aspect of the present invention is the capability of detecting an empty vessel condition. The method includes the steps of establishing a measuring length which is less than the reference end of probe location, and establishing a maximum probe length which is greater than the reference end of probe location. An empty vessel condition is detected when the end of probe location is greater than or equal to the measuring length, the end of probe location is less than or equal to the maximum probe length and no level reflection is detected.
Additional objects, advantages and novel features of the invention are set forth in the description that follows, and will become apparent to those skilled in the art upon reviewing the drawings in connection with the following description.